This may be caused due to document inclination.
To reduce the amount of the black line, hold lightly the right and left edges of the paper like fig. 1 or 2 to ensure the paper is scanned without inclination.
Keep holding the document while the paper is fed. Do not hold the paper with too much force; otherwise it may results in paper jam.
FIGURE 1. fi-7600/fi-7700/fi-7700S
FIGURE 2. fi-7460/fi-7480